Autotransformer for alternating current having variable transformation ratios



June 24, 1930. s TT 1,767,947

AUTO TRANSFORMER FOR ALTERNATING CURRENT.

HAVING VARIABLE TRANSFORMATION RATIOS Filed April 18 1927 Patented a 24, 1930 UNITED STATES PATENT OFFICE PIETBO BTRAGIOT'II, OII' TURN, ITALY, ASSIGNOR TO FEDERICO PAGLIANI, OI TUBIN,

ITALY AUTOTRANSI'OBMEE FOB ALTEBHATING 0111mm name VARIABLE rmsroammpu m'rros 1 Application filed April 18, 1927, Serial No. 184,735, and in Italy April 22, 1926.

This invention relates to auto-transformers, having variable transformation ratios, for alternating current, and has for its object an improved transformer of this character. According to the principal feature of the invention an auto-transformer is provided with four windings connected electrically in series and arranged individually in pairs on magnetic cores in such a manner as to generate interlinked magnetic fluxes therein, the two inner windings which constitute one of the airs, being arranged to generate magnetic uxes of equal magnitudes acting in opposite directions, and the two outer windings, constituting the other pair, being arranged to generate fluxes of equal magnitudes acting in the same direction, when a current traverses. the four windings in series, means being provided for the relative orientation of the magnetic fluxes generated by the two pairs of windings, with the object of varying the difference of potential between the middle point of the series of windings and the two terminals of the series.

A further feature of the invention consists in the application of an auto-transformer of the kind described above to the various cur rent tappings of an ordinary transformer for the supply of an alternating current line with the object of regulating the working tension of the line.

The invention is illustrated diagrammatically and by way of example only, in the accompanying drawing in which :Figures 1 and 2 are transverse and longitudinal sec tional views respectively of the improved auto-transformer. Figure 3 a circuit diagram thereof. Figure 4 an equivalent circuit diagram, and Figures 5 and 6 are views illustratmg the application of the auto-transformer to the regulation of the working tension of a line supplied by an ordinary transformer. t

Referring, first to Figures 1 and 2 of the drawing, the auto-transformer comprises two magnetic cores 1, 2 built up of iron laminae, 1n WhlCh. su table recesses are provided for accommodating the various wind-i ings. The magnetic cores which are rated from one another by a small air-gap, are rotatable with respect to one another, for example the core 1 is the stator (as illustrated in the drawing), being integral with the body portion 5, and the core 2 is the rotor, being integral with a shaft 2 rotatably mounted in bearings 3, 4 of the body portion 5. The rotor may be rotated directly by hand or mechanically by means of a worm 6 adapted to engage with a worm wheel'7 secured to the shaft 2. The windings (Figures 1, 2, 3) are four in number, I, II, III, and IV, and are connected together in series, preferably an equal number of turns being provided in each winding. The windings I and IV are integral with the stator and the windings II, III are integral with the rotor. The terminals a, g of the series of windings are connected to two wire clamps X, Z, the middle point d of the series, being connected to wire clamp ,Y. ,The windings I and IV, when a current traverses them, generate a sepaflux interlinked with the rotor in the fixed direction N, N. When a current from X to Z or vice versa traverses the windings I and IV, in the same direction, the combined action of the windings is to generate two e ual fluxes resulting in a single resultant flu'x, ut, if on the contrary the current traverses the windings I and IV in opposite directions and converges and leaves at the middle point d-, the combimd action of the two windings is such as to generate two opposite fluxes resulting ina flux which'is equal to the difference between the fluxes enerated by each of the windings I and IV. lhe windings II and III are arranged in such a manner that each generates a fiux in the direction M, Mof the rotor and interlinked with the stator. The combined action of the windings II, and III when they are traversed by a single current in the same direction, is to generate equal and opposite fluxes, and when they are traversed by the current in different directions, to generate fluxes in the same direction which combine to form a single resultant flux. In other words the windings I and IV are wound in the same direction, whilst the windings II and III are wound in a direction opposite to one another.

The rotor 2 may be rotated from the position in which it is illustrated in Figure 1 through an angle a (Figure so that the direction M, M of the field generated by the rotor windings is inclined at an angle a to the direction N, N of the field generated by the windings of the stator. In this case the alternating field in the direction M, M has two orthogonal components, one in the direction N, N and the other at right angles to this direction, and in transformers of a certain power it is advisable to eliminate the component at right angles to the direction N, N by means of a short circuited winding IV of low ohmic resistance arranged on the stator (Figure 1).

In practice, if the rotor is not rotated through an angle greater than one complete turn in either direction from the middle position, the connections b, cand e, f between the circuits of the stator and rotor, and also between the middle point -(i and the wire clamp Y, may be eflected through flexible conductors, but there is no reason why these connections should not be made throng friction contacts with the object of increasalent ing the range of operation. Thus, as shown in Figure 2, the terminals of the windings of the rotor and the middle point of these windings are connected respectively to rings -c, e, dintegral with the shaft 2 of the rotor, the brushes b, f making contact with the rings 0, 6- being connected to the internal ends of the circuits I and IV, and a brush Z making contact with the rin -d-, being connected to the wire clamp The action of the transformer is as follows- Assume that the rotor (Figure 3) has been rotated through an angle a from its normal position in which it is illustrated in Figure l and that an alternat' flux (1:, generated by a difference of alternating potential V impressed at the ends X, Z of the series of windings, is established in the direction N, N; then if S is the number of the turns in each winding, in the winding I there will be S turns linked with the flux In the winding II there will be S turns linked with the component cos a of this flux, or its equiv- -S cos 11 turns linked with the flux In the winding III there will be S cos a turns linked with the flux +4 and in the winding IV, S turns linked with the flux The equivalent circuit is illustrated in Figure 4, that is to say an autotransformer the total number of turns of which is 28 and in which the position of the wire clamp Y is as S cos 11 turns from the middle of the winding. The turns of the two circuits of this auto-transformer would therefore be respectively S (1 cos a) and 2S, and consequently the ratio of transformation is this ratio varying from one to zero when a varies from zero to 1r.

Figure 5 illustrates the ap lication of an auto-transformer of the kind escribed above for regulating the tension in the line 8, 8 supplied for example by the secondary 9 of a transformer. With the object of regulating the tension of a line the secondary 9 has been provided previously with a number of tappings n2, n-1, n, n+1 etc., at various points in the secondary of the transformer, the line wire 8' being arranged to be connected to any of these tappings to increase or diminish the working tension of the line. It is however a diflicult matter to eflect connection of the line wire 8' with the various trappings, when the whole system is under load and comparatively great currents are to be dealt with, commutators immersed in oil having to be employed for this purpose. Moreover the tension is not regulated in a continuous manner by such an arrangement but in steps corresponding to the difference of potential between the various tappings.

IVhen employing an auto-transformer constructed in accordance with the present invention, the wire clamps X, Z are connected to two successive tappings, the wire clamp Y being connected to the line wire 8. In certain circumstances the secondary winding consists of two portions 9, 9' (see Fig. 6), the tappings being arranged toward the middle of the secondary winding, that is to say, at the internal end of one of the two portions of the winding, and when this is the case the wire clamp Y is connected to the internal extremity 9 of the portion 9' of the winding not comprising the tappings, while the external extremities 9, 9' of the two portions of the winding are connected to the line wires 8 and 8. Thus for an angle 11 equal to zero, the fields generated by the windings I, II eliminate one another, those generated by windings III, IV being additive. The reactance offered to the passage of a current --z" between X and Z is at a minimum, but that offered to the passage of a current i"- between Z and Y is com aratively great, and consequently i" wi 1 be zero, and only a small In etization current will pass; the; tential 0 wire clamp Y will be substantia y equal to that of the tapping n-1 and consequently of wire clamp X. Upon making a equal to 1r the fields generated by windings I and II will be additive and those due to III and IV will eliminate one another; the reactance ofiered to the passage of the current i" is therefore negligible, that ofiered to the p of the current i' being comparativelyYgi-eat, so that the potential of wire clamp will be substantially that of the tapping 4 and consequently ofZ. Therefore by varying a from zero to 1r the value of the tension of line can be varied in a continuous manner between the values of the potential of the tappings n--1 and 10. Moreover when it is desired to disconnect from one tapping and to connect to the succeeding tap the rotor is placed in a position,in w b i izi a is equal to zero or to r, so that the whole of the current will pass across the tapping which remains connected, only the m etization current therefore p throng: the tapping which is about to be connected or disconnected. simtfilicity of operation, without any danger, is us attained and commuta tions may thus be effected by means of an ordinary controller 'no special commutator-s immersed in oil which are very expensive and cumbersome I s-ac:

It will be evident that the invention is not limited to'the articular-arrangements hereinabove descri ed which maybe varied, as desired to suit particular requirements.v

I claim: 1

1. An auto-transformer ha i variable transformation ratios, com our windings electrically connec in serles, a core for thepair of intermediate'windings and a core for the. pair of outer win of the series, the windings of the iuterm ate pair to generate ual and of windin arranged opposite uxes an the windings o the external pair being arranged to generate equal fluxes in the same direction when the current traverses all four win in series, and means for vaif'iying the relatlve orientation of the magnetic uxes generated by the two pairs of windings to vary the difference of potential between the middle point of the series connected windings and the two terminals thereof.

3. An auto-trans zi'mer havin variable transformation ratios,- comprising our windings electrically connected in series, two magnetic relatively movable cores, the intermediate windingsnarranged on one core and the extreme win gs arranged on the other core, a short circuited winding of low ohmic resistance on one of the cores at right angles to the windings thereon to oppose any variation of flux in the other core in a direction at right angles to the flux generated by the windings mounted on the first core. 1

4. Thecombination with a stationary alternating current transformer having a set of taps; of an auto-transformer having two coruintwo windings on each core, all the windings ing connected in series, and one of the cores rotatable with respect to the other, the intermediate windings being wound in opposite directions and their middle point arranged for connection to a service line and the external windings connected between two taps of the stationary'transformer.

5. The combination with a stationary alternating current transformer having a portion with an intermediate set of taps and another portion without taps; of an auto-transformer ha two cores, two windings on each core, all the being connected in series, and one of the cores rotatable with respect to the other, the intermediate windings being wound in opposite directions and their middle point connected to the line through an invariable portion of the winding of the stationary transformer and the external windings connected between two taps of said stationary transformer.

In testimony that I claim the foregoing as my invention, I have si ed my name.

- PIETRO STRAGIOTTI. 

